Macromolecules 1993,26, 1180-1182
1180
Role of Trimethylaluminum on the Zirconmns-Methylaluminosane-Catalyzed Polymerization of Ethylene'
to a dramatic incream in catalyst productivity ae well ae catalyst lifetime. This paper reports the resulte of this study.
Experimental Section 8. Srinivara Reddy, 0. Shaahidhar, and S. Sivara" Division of Polymer Chemistry, National Chemical Laboratory, Pune 411 008, India Received June 30,1992 Revised Manuscript Received November 2, 1992
Introduction Homogeneous catalystsbaaed on group IV metallocenemethylaluminoxane (MAO)system have been the subject of intensive research in recent years.' They are also currently attracting sisnificant commercial attention2In spite of many advances, the role of cocatalyst continues to be intriguing. A question that still awaits a satisfactory answer is the precise role of free trimethylaluminum (TMA)associatedwith MA0 on the activity and the kinetic profile of the catalyst. MAO,whose structure ispostulated asa mixture of linear and cyclic oligomers of [Al(Me)OI, units, is prepared by controlled hydrolysis of TMA with hydrated metal salts. Depending on the nature of salt used and the reaction conditions, MA0 with widely differing activities as cocatalysts in polymerization of ethylene is f ~ r m e d .Sinn ~ and co-workers proposed that MA0 forms an adduct with three molecules of TMA which even after distillation in the presence of cumene or complexation with tetrahydrofuran retained one molecule of strongly associated TMAe4 There have been only a few attempts in the literature to examine the role of associated TMA on the nature of catalysis. Reeconiand co-workersshowed that TMA alone can initiate ethylene polymerization with various zirconocenes.5 However, activities and molecular weights were much lower. The activities could be substantially enhanced by using sterically hindered zirconocene and triisobutylaluminum.6 Giannetti and co-workers studied the effect of free TMA on polymer productivity and the molecular weight of ethylene polymerization using (Indh&(CH3)2 and Cp2Zr(CH2CsH& and MA0 at [All/ [Zrl= 16600 and at 60 OCe3 The ratio of free TMA to MA0 varied between 0 and 1.2. Catalyst activity was found to decreasewith increasing ITMA]/ [MA01ratios. Molecular weights of the polyethylene obtained with Cp2Zr(CH2C6H5)2 strongly decreased with increasing TMA whereas it was invariant when (Ind)zZr(CH& was used. Ethylene polymerization with Cp&ClJMAO was investigated by progressively replacing MA0 with TMA at [AlMZrI = 1970.7 The ratio of [Allw to [ A l l ~was o varied between Oand 1OOO. WithincreasingadditionoffreeTMAtoMAO, both catalystproductivity and molecular weight decreawd. The kinetic profile of the catalyst was not substantially altered up to a [TMAI/[MAOl I10. At [TMAl/[MAOl = 100,the rate profile changed from a decay to a buildup curve. The interaction of CpzZrCl2 with MA0 has been studied by Cam and Giannini using lH NMR spectroscopy.s They concluded that, at [All/[Zrl> 15,Cp,ZrClZ m completely monoalkylated by the TMA present in MAO. In the course of a program of remarch in homogeneous catalyet for ethylene polymerization we have found that addition of TMA to MA0 under specified conditionsleads + NCL
Communication No. 5565. 0024-9297/93/2226-1180$04.00/0
All operations were carried out under dry nitrogen using standard benchtop inert atmosphere techniques. Material. Toluene (LOBA-AEt grade) wm purified by refluxing over sodium wire and subsequent distillation under nitrogen. Polymerization grade ethylene wm obtained from the C2-G gas cracker unit of Indian Petrochemical Corp. Limited at Nagothane (02= 3 ppm, moisture = 4 ppm). TMA (ScheringA.G., Germany) and zirconooene (AldrichChemicals) weme used as received. A sterically hindered aryl oxide substituted alkylaluminum [AlMe(BHT)d was prepared from TMA apd 2 , M tert-butyl-p-cresolas per literature procedure and c-rized by melting point, lH NMR,and methyl/nluminum ratio? MA0 (Schering-AG.,Germany),Sb% wtAlintoluenedution (averagemolecular weight = 850)wm found to have a [methyl]/ [aluminum] ([Me/AlI) ratio of 1.221. Toluene was W e d off at 30 OC/O.S mmHg which resulted in a glassy solid. The [Me]/ [All ratio of the solid was 1.20:l. The solid WBB analyzed for free TMA by NMR in toluene-ds and was found to be 12% .I A stock solution of solid MA0 in toluene was prepared and used for further study. Polymerization. Ethylene polymerizationwas conducted at atmospheric pressure and at 70 "C in a jacketed reaction cell connected to a l-Lgas buret. The desired amount of TMA was first added followed by MA0 to 30 mL of dry toluene saturated with ethylene. The reaction was initiated by addition of zirconocenesolution. The temperature was maintained constant both in the reaction cell and in the gas buret by circulating water through the jacket. Consumed ethylene was measured as a function of time. The reaction was terminated by addition of acidified methanol, fiitered,and dried under vacuum to a constant weight. Analysis. The aluminum content was estimated by EDTA titration. The methyl content was determined by estimating the amount of methane evolved upon hydrolysis usingagm buret NMR analysis was performed on a Bruker AC-200 spectrometer. Intrinsic viscosities of the polymers were measured at 135 "C using decalin as solvent.
Results and Discussion Polymerizationof ethylene was carried out with three differentcatalystsystem, namely, Cp2zrcldTMA (I),C ~ T ZrClJMAO (111, and Cp2ZrCl&fAO + TMA (III) in toluene at 70OC. For the sakeof comparison, AlMe(BHT)2 was also used instead of MAO. TMA was added to a toluene solution of AlMe(BHT)2just prior to addition of Cp2ZrC12. The zhnocene concentration ranged from 1.15 x 10-5to 1.15 X moVL. Three ratioa of [Allt,d[Zrl were studied which in the case of TMA and MA0 were 103,104,and 106. In the case of experiments with MA0 + TMA, [Allt&[Zr] varied from 1676 to 167 600. However, the free TMA added to MAO, by itaelf, contributed to an [AlI/[Zrl of 109, l@, and loa, which is essentiallysimilar to the ratio used when TMA alone was used as cocatalyst. The resulta are shown in Table I and are graphically plotted in Figures 1-3. The Cp2ZrClJTMA system shows relatively poor catalyst productivity and low initial rates of polymerhation. Both the rate and catalyst productivity i n d with increasing [Al]/[Zr] ratio. The CplZrCldMAO system shows a s i m i i behavior except that absolute magnitudes of both the rate and catalyst productivity are higher. Addition of TMA causes a dramatic increase in catalyst productivity (entries 7-9). The changes in the kinetic profile of the polymerhation (Figures1 and 2)upon addition of TMA to MA0 are more 0 1993 American Chemical Society
Macromolecules, Vol. 26, No. 6, 1999
Notee 1181
Table I Ethylene PolymerixationUdng Zirconocendrgmoalu"
entry no.
Al compd
[Mel/[All
1 T M A 2 T M A 3 T M A 4 MA0 5 MA0 6 MA0 7 MAO+TMA 8 MAo+TMA 9 MAO+TMA 10 AIMe(BHT)Z+TMA AlMe(BHT)z+ TMA 11 0
3.00 3.00 3.00 1.22 1.22 1.22 2.20 2.20 2.20 2.20 2.20
1 0 2 [ A l h ~ 107[Zrl 1.15 115.00 1.15 11.50 1.15 1.16 1.15 115.00 1.15 11.50 1.15 1.15 1.15 + 0.78 115.00 1.16+ 0.78 11.50 1.15+0.78 1.15 1.15+0.78 115.00 1.15+ 0.78 11.50
[Allt&[Zrl lo00 loo00 1 m lo00 loo00 1OOOOO 1676 16760 167800 1676 16760
Compounds.
time (min)
(M-8-9
[keof PE/@of %h)l
120 120 120 9 30 60 50 120 120 120 120
0.66 2.50 5.65 12.87 68.24 89.81 20.65 52.14 62.87 2.04 7.50
3.94 7.21 13.22 4.7% 20.51d 136.20 14.W 77.85 205.31 3.24 6.57
10%~
productivityof
[VI*
(dLg9 1.59 1.41 1.18 2.31 2.67 1.57 1.69 2.50 1.48 1.66
Polymerization conditions: temperature, 70 O C ; toluene = 30 mL; PcJ+, = 1 atm. * In d d i at 135 OC. Reaction CBaSed after 9 min. ceased after 30 min. Reaction ceased after 50 min.
d Reaction
s'Or-----
c
5.0
TIME (MINI
Figure 1. Polymerization of ethylene using zirconocenetrimethylaluminumcatalyat: (I) [ A l l d [ Z r l = 103; (11) [ A l I d [Zrl = l(r; (III) [ A l l d [ Z r l = 105.
revealingof the subtle yet substantiverole played by TMA. TMA alone with Cp2ZrCl2 shows a steady-state buildup type kinetic curve, especially at a [All/[Zrlratio of 103 and 104. The onset of a moderate rate decay is evident at [All/[Zrl = 106 (Figure 1). MA0 alone shows a decay type kinetic curve, the rate of decay being faster at lower [All/[Zrl ratios (Figure 2). The peak rate is achieved within 6-10 min, and therefore rapid decay of the rate occurs. Decay type kinetic curves for ethylene polymerization using Cp2ZrC12/MAO have been reported earlier in the literature.'J@12 Addition of TMA to MA0 slightly reduces the peak rate. However, with addition of TMA, the kinetic profile changes from a decay typeto a buildup type curve. At a [Al],d[Zr ratio ] of 167 OOO (curve VI), the rate shows negligible decay, exhibiting essentially a steady-state kinetic behavior for over 2 h. To the best of our knowledge, such a beneficial effect of added TMA in a MAO/metaUocenecatalyst system from the point of view of practical catalysis is not explicitly evident in the prior literature. Interestingly and contrary to data published in prior literature, moleculr weight is only marginally d e p d by addition of free TMA to MAO,under the conditions of this study. We also examined the behavior of a sterically hindered aryloxy derivative of TMA, namely AlMe(BHT)2, as cocatalyst for ethylene polymerization. AlMe(BHT12 can be ieolated as a crystalline solid, free of complexed TMA. In conjunction with zirconucene, a toluene solution of AIMe(BHV2failed to initiate polymerizationof ethylene.
"0
20
40
60
80
100
120
140
TIME (MINI
Figure 2. Polymerization of ethylene using airconmneMA0 and zirconocene-MA0 + TMA catalysk (I) [Allh~o/[Zrl=109; (11) [AllM.4o/[Zrl= 1% (m) [AllMAo/[Zrl= l* (Iv)[Alltod [Zrl = 1676; (V) [ A l l d [ Z r l = 16760; (VI) [All&[Zrl = 167 600.
However, upon addition of TMA (entries 10 and 11)the catalyst showed pronounced activity. There was a small but definite enhancement of initial rate over the TMA/ zirconocene catalyst. A decay type kinetic profile, reminiscent of MAO,was observed (Figure 3). These resulte have significant implications on the metallocene-methylaluminoxane-catal& polymerization of olefins. They establish that the [Mel/[All ratio
inMAOneedetobesuitablyadjustedtoproduceaca~t of high activity and long kinetic lifetime. This result ale0 has a bearing on the choice of syntheais conditions for obtained MA0 with a defined [Me]/ [All rati0.9.'~ Apart from contributingto catalyst activity,TMA playa themore important role of stabilizing the active centers and extending catalyst lifetime. A sterically hindered aryl oxide of TMA, which is monomeric in nature and beare a close similarity to the structure of oligomeric aluminoxane, also exhibita a kinetic behavior similar to that of MAO. However, catalyst productivity is far inferior.
Macromoleculee, Vol. 26, No.6, 1W
1182 Notee 7.0
I
I
I
I
I
I
1
Acknowledgment. S.S.R.and G.S.thank the CSIR, New Delhi, for a reeearch fellowship.
References and Notes
Figure 3. Polymerization of ethylene wing zirconmen-AlMe(BHT)*+ TMA catalyst: (I) [All,d[Zrl = 1676;(11) [AlI,d [Zr]= 16 760.
Nevertheless, this raises the hope that a simple organoaluminum compound can be found that mimics the essentialfeaturea of the morecomplexMAO-basedcatalyst system. Systematic research, in pursuit of this goal, is currently underway in our laboratory.
Kamineky, W.; Buechermohle, M. In Recent Aduancea in Mechanistic and Synthetic Aspects of Polymerization; Fontanille,M., Guyot,A.,Eds.; D. RsidelPublishing Co.: Holland, 1987;p 503. Leaversuch, R. D.Mod. P h t . Znt. 1991,Zl (lo),34. Wood, A.; Chynoweth, E. Chem. Week 1992,160(19),52. Giannetti, E.;Nicoletti, G. M.;Mazzocchi, R.J. Polym. Sci., Polym. Chem. Ed. 1986,23,2117. Sinn,H.; Bliemeister, J.; Cladtzar, D.; Tikwe, L.; Winter, H.; Znrncke, 0.In TramitionMetab and Orgunomstallics08 Catalysta for Olefin Polymeritotiom; Kaninaky, W., Sinn, H., Eds.; Springer-Verlag: New York, 1988,p 267. Resconi, L.;Boesi, S.; Abis, L. Macromolecub 1990,25,4489. Resconi,L.;Giannini, U.; Albmti,E.;Piemonhi, F.; Fiord, T. Polym. Prepr. (Am.Chem. SOC.,Diu. Polym. Chem.) 1991, 32.463. Chien, J. C. W.; Wang,B.-P. J. Polym. Sci.,Polym. Chem.Ed. 1988,26,3089. Cam,D.;Giannini, U. Makromol. Chem. 1992,193,1049. Healy, M.D.;Wierda, D. A.; Barron, A. R.Organometallics 1988,7,2543. Mallin, D.T.;Rauach, M.D.; Chien, J. C. W. Polym. Bull. 1988,20,421. Tsubui, T.;Kashiwa, N. Polym. Commun. 1988,29,180. Kaminsky,W. InHistory ofPolyolefim;Seymow,R.B.,Cheng, T., Eds.;Reidel Publishing Co.: Holland, 1986; p 257. These results indicate that data from published literature, especially originating from different laboratoriee, must be compared with caution. Only in a very few inetancea is complete characterization of MA0includingthe TMA content explicitly indicated in published papers or patenta.
